@Smohith ***********
, There are some offices that only accept cancellations on the effective day, not in advance. In that case, you might not have many choices. Possibly,
1. As mentioned in the mail, ask your employer to set the due date as 1 April. Apply cancellation on that day before you fly out.
2. Apply for cancellation on 29 March. Immediately apply for a tourism (or whatever reason) extension for a 1,900 baht fee to receive an "application denied, up to 7 days to leave the kingdom" stamp. (Bring your air ticket).
3. Apply for cancellation on 1 April, then leave. (<-this part I'm not sure whether you could have hard time or it is the same procedure as cancellation within the time, or else) You may or may not be fined, but you will get an overstay stamp.